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Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas

Abstract

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (≥2,000 °C)1. Solid silicon has recently been generated directly from silica at much lower temperatures (≤850 °C) via electrochemical reduction in molten salts2,3. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants2,3. Here we demonstrate a low-temperature (650 °C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m2 g-1), and contained a significant population of micropores (≤20 Å). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological4,5,6 or synthetic silica templates7,8,9 for sensor, electronic, optical or biomedical applications10,11,12,13.

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Figure 1: Shape-preserving magnesiothermic reduction of silica diatom frustules.
Figure 2: Phase content of diatom frustules after magnesiothermic reduction.
Figure 3: Fine-scale structure of silicon replicas produced by magnesiothermic reduction of Aulacoseira diatom frustules.
Figure 4: Gas sensor based on a silicon frustule replica.

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Acknowledgements

This work was supported by the US Air Force Office of Scientific Research (H. C. DeLong and J. Fuller) and the US Office of Naval Research (M. Spector). We thank M. Bestor for assistance with XPS analysis and S. Yoo for help with focused ion beam milling.

Author Contributions Z.B., M.R.W., S.M.A., P.D.G., M.B.D. and K.H.S. conceived, developed and demonstrated the low-temperature magnesiothermic reduction and selective dissolution process. S.S. prepared and tested silicon replica gas sensors. G.A. cultured diatoms for conversion. Transmission electron microscope and BET analyses were conducted by Y.C. and B.C.C., respectively. H.W.A. and M.L. conducted and analysed FTIR measurements. Z.K. and C.J.S. conducted and analysed photoluminescence measurements. M.B.D. conducted fluorescence microscopy. Overall data analyses, project planning and paper preparation were largely conducted by Z.B. and K.H.S.

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Correspondence to Kenneth H. Sandhage.

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Supplementary Figures and Methods

This file contains Supplementary Figures S1-S5 with Legends and Supplementary Methods. Figure S1 reveals silicon-converted Melosira nummoloides frustule replicas. Figures S2 and S3 provide XPS and FTIR confirmation of silicon in Aulacoseira frustule replicas. Figure S4 reveals the significant micropore population in silicon replicas of Aulacoseira frustules. Figure S5 demonstrates the photoluminescence of silicon Aulacoseira frustule replicas after immersion in water. The equipment and conditions used to obtain the secondary electron, transmission electron, and fluorescence microscope images of silica and silicon-converted diatom frustules are described in Supplementary Methods. (PDF 1127 kb)

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Bao, Z., Weatherspoon, M., Shian, S. et al. Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas. Nature 446, 172–175 (2007). https://doi.org/10.1038/nature05570

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